Schistosomiasis (Schistosoma)

Etiology

Schistosoma organisms are the trematodes, or flukes , that parasitize the bloodstream. Five schistosome species infect humans: Schistosoma haematobium, S. mansoni, S. japonicum, S. intercalatum, and S. mekongi . Humans are infected through contact with water contaminated with cercariae , the free-living infective stage of the parasite. These motile, forked-tail organisms emerge from infected snails and are capable of penetrating intact human skin. As they reach maturity, adult worms migrate to specific anatomic sites characteristic of each schistosome species: S. haematobium adults are found in the perivesical and periureteral venous plexus, S. mansoni in the inferior mesenteric veins, and S. japonicum in the superior mesenteric veins. S. intercalatum and S. mekongi are usually found in the mesenteric vessels. Adult schistosome worms (1-2 cm long) are clearly adapted for an intravascular existence. The female accompanies the male in a groove formed by the lateral edges of its body. On fertilization, female worms begin oviposition in the small venous tributaries. The eggs of the 3 main schistosome species have characteristic morphologic features: S. haematobium has a terminal spine, S. mansoni has a lateral spine, and S. japonicum has a smaller size with a short, curved spine ( Fig. 326.1 ). Parasite eggs provoke a significant granulomatous inflammatory response that allows them to ulcerate through host tissues to reach the lumen of the urinary tract or the intestines. They are carried to the outside environment in urine or feces (depending on the species), where they will hatch if deposited in freshwater. Motile miracidia emerge, infect specific freshwater snail intermediate hosts, and divide asexually. After 4-12 wk, the infective cercariae are released by the snails into the contaminated water.

Epidemiology

Schistosomiasis infects more than 300 million people worldwide and puts more than 700 million people at risk, primarily children and young adults. There are 3.3 million disability-adjusted life-years (DALYs) attributed to schistosomiasis, making it the 2nd most disabling parasitic disease after malaria. Prevalence is increasing in many areas as population density increases and new irrigation projects provide broader habitats for vector snails . Humans are the main definitive hosts for the 5 clinically important species of schistosomes, although S. japonicum is also a zoonosis, infecting animals such as dogs, rats, pigs, and cattle. S. haematobium is prevalent in Africa and the Middle East; S. mansoni is prevalent in Africa, the Middle East, the Caribbean, and South America; and S. japonicum is prevalent in China, the Philippines, and Indonesia, with some sporadic foci in parts of Southeast Asia. The other 2 species are less prevalent. S. intercalatum is found in West and Central Africa, and S. mekongi is found only along the upper Mekong River in the Far East.

Transmission depends on water contamination by human excreta, the presence of specific intermediate snail hosts, and the patterns of water contact and social habits of the population ( Fig. 326.2 ). The distribution of infection in endemic areas shows that prevalence increases with age, to a peak at 10-20 yr old. Exposure to infected water starts early in life for children living in endemic areas. Passive water contact by infants (accompanying mothers in their daily household activities) evolves to more active water contact as preschool and school-age children pursue recreational activities such as swimming and wading.

Measuring intensity of infection (by quantitative egg count in urine or feces) demonstrates that the heaviest worm loads are found in school-age and adolescent children. Even though schistosomiasis is most prevalent and most severe in older children and young adults, who are at maximal risk for suffering from its acute and chronic sequelae, preschool children can also exhibit significant disease manifestations.